1. Field of the Invention
This invention relates to a waveform shaper and a sigma-delta (.SIGMA..DELTA.) D/A converter that generate a high-accuracy analog output through correction of clock jitter.
2. Prior Art
In general, when oversampling is carried out, as the sampling frequency is higher, the quantization noise is dispersed over a wider frequency region to lower the noise spectrum level in each unit frequency band, and accordingly the signal-to-noise ratio in a signal frequency band is improved. A sigma-delta D/A converter conventionally employs a combination of a known oversampling technique and a technique called "noise shaping". The technique of noise shaping largely reduces noise level in an audible frequency band by shaping the quantization noise, i.e. changing the noise spectrum of quantization noise from a flat response (white noise) to a response where the noise level is low at lower frequencies and high at higher frequencies.
In the sigma-delta D/A converter, a low-pass filter is provided at a later stage thereof for removing higher frequency components than the frequency band of a fundamental wave. To prevent clock jitter from adversely affecting the filtering operation, the low-pass filter is conventionally integrated in an LSI in the form of a switched capacitor circuit. Further, an sigma-delta A/D converter is also constructed using a switched capacitor circuit.
The switched capacitor circuit performs switching of the supply of current to a capacitor thereof to thereby use the capacitor as an equivalent of a resistor. Therefore, to integrate the switched capacitor circuit in an LSI, a capacitor is required to be provided within the LSI. Such a capacitor is formed by reverse bias of a PN junction and a capacitance between two poly-silicon layers or between a poly-silicon layer and a metal layer, the capacity of which is dependent on the surface area of the associated layers. Accordingly, when the switched capacitor circuit is integrated within the LSI, the capacitor occupies a large area, jeopardizing reduction of the chip size and higher-density integration of the integrated circuit.
On the other hand, when the sigma-delta D/A converter is constructed by integrating a waveform shaper and an active filter as a low-pass filter in an LSI, instead of using the switched capacitor circuit, waveform shaping has to be carried out by an internal clock which is not free from jitter, so that the pulse width of one-bit pulse data from the sigma-delta modulator varies to cause noise or waveform distortion. These problems make it difficult to integrate the sigma-delta D/A converter in an LSI. Therefore, it is a conventional technique that component parts of the sigma-delta D/A converter up to the sigma-delta modulator are integrated in the LSI, and output pulses from the sigma-delta modulator are subjected to waveform shaping outside the LSI by using an external clock with small jitter, followed by filtering the shaped pulses by an active filter or the like. This requires, however, an external circuit in addition to the LSI, so that the size and cost of the sigma-delta D/A converter are increased.